1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly to a surface emitting type light-emitting device that has a p-n junction.
2. Description of the Prior Art
Light-emitting devices are used in optical printers that use a beam of light to record information, in bar-code image reading systems that utilize the intensity of a reflected beam of light, and in optical communications devices that utilize optical signals.
FIGS. 7 and 8 show the structure of a conventional light-emitting device. With reference to the drawings, GaAsP containing tellurium is used to form an n-type GaAsP semiconductor layer 2 into which zinc is diffused to form a p-type GaAsP semiconductor layer 1. A positive electrode (p-electrode) 3 is then formed on the upper surface of the p-type GaAsP semiconductor layer 1 and a negative electrode (n-electrode) 4 on the lower surface of the n-type GaAsP semiconductor layer 2. When a forwardly biased voltage is applied to the junction between the p-type GaAsP semiconductor layer 1 and the n-type GaAsP semiconductor layer 2, the electrical energy is converted into optical energy by the recombination of the minority carriers injected into the p-type GaAsP semiconductor layer 1 and n-type GaAsP semiconductor layer 2 with the majority carriers, producing an emission of light.
However, when a forwardly biased voltage is applied to a p-n junction, the relationship between the current flowing in the light-emitting device and the optical output is as shown by the solid line in the graph of FIG. 9. That is, in low current regions the increase in optical output power relative to the increase in current is not linear but is instead strongly dependent on the degree of current increase. In FIG. 9 the broken line is a straight line that indicates a constant, or linear, relationship between the increase in current and the increase in optical output power.
That is, when a voltage V is applied to the p-n junction, the device current is divided into a component that is proportional to exp(eV/kT) (hereinafter referred to as the type A current component) and a component that is proportional to exp(eV/2kT) (hereinafter referred to as the type B current component), and the intensity of light emitted by the light-emitting device is proportional to the type A current component. In a high current region the type A current component accounts for the major portion of the total current, so the increase in the optical output power of the light-emitting device is linear with respect to the increase in the current. However, when the current is small the type B current component accounts for a large proportion, and the increase in light intensity relative to the increase in current becomes nonlinear.
In a light-emitting device, an increase in light intensity that is nonlinear with respect to the increase in current presents problems when it comes to using the current to control the light intensity, making it difficult to control the light intensity when such light-emitting devices are used as light sources of optical printers, image readers and optical communications devices.